1. Field of Invention
The present invention relates to a composition for photon energy up-conversion, a system comprising said composition and to uses of said composition and said system.
2. Discussion of the Background
In a number of systems, it has been observed that irradiation by light with longer wavelength causes emission of a light with shorter wavelength. This phenomenon, which is also related to as “frequency up-conversion” or shortly “up-conversion” is most often associated with high light intensities available from pulsed lasers. It is presently believed that the up-conversion process involves the energy transfer of several lower excited states to a single higher excited state which is then capable of emitting light with a shorter wavelength, i.e. higher energy. This process has been described for a number of inorganic systems in the solid state, including crystals, thin films and nanoparticles. Usually the up-conversion process in the crystalline systems includes “sensitising” components, “activator” components and/or matrix (crystal) components. Typically the matrix is co-doped with rare earth ions, which act as “sensitisers” as well as “activators”. One of the dopands absorbs in the low-wavelength region (typically infrared) whereupon the absorbed photon energies are then transferred to another dopand, the activator ions, which emit in the blue or green region (E. L. Falcao-Filho et al. J. Appl. Phys 92, 3065, (2002), Yoh Mita et al. Appl. Phys. Lett. 62, 802 (1992), Trash et al. Journal of OSA Bulletin 11, 881 (1994)). Furthermore crystalline nanoparticles have been described, a combination of which is dispersed in host matrices to form thin films. These crystalline nanoparticles also have been shown to be capable of energy up-conversion, which process takes place between the various species of nanoparticles and include an energy transfer step (e.g. U.S. Pat. No. 6,541,788) or the crystalline nanoparticles act as a matrix for other dopands such as Eu3+-ions, Tb3+ions, Tb3+-ions, Ce3+-ions etc., and these dopands upon irradiation of light are capable of increasing the fluorescence intensity and quantum efficiency of the nanoparticles.
These systems are of potential interest for the fabrication of lasing materials, photovoltaic devices and so on. Due to the nature of the components involved they are, however, rather expensive in manufacture and furthermore not particularly suited for the preparation of for example, films over large areas or the preparation on flexible substrates, both of which should be particularly useful for the fabrication of commercially useful photovoltaic devices, such as solar cells.
One approach to address this problem was to use organic compounds, instead of inorganic ones. These organic up-conversion systems are all based on direct two-photon or multiphoton excitation and/or the excitation of molecules populating high vibrational states into a first excited state, which latter process is also sometimes referred to as “hot band-absorption”. In the direct, i.e. simultanous two-photo excitation the up-conversion is a result of a direct two-photon pumping of dyes with large two-photon absorption (TPA) cross-section, which dyes are either in solution or in films (including so called “solid” solutions, with inert polymers as an inactive matrix, i.e. a solid “solvent”). This inactive matrix is inactive in the sense that it does not take part in the up-conversion process whatsoever. Various systems have been described, and there is an ongoing search for new organic dyes with greater TPA-cross-section and TPA-dyes which are bound to polymer molecules or doped in polymer matrices (U.S. Pat. No. 5,912,257, U.S. Pat. No. 6,555,682, U.S. Pat. No. 6,100,405, T. Kojei et al. Chem. Phys. Lett. 298, 1 (1998), G. S. He et al., Appl. Phys. Lett 68, 3549 (1996) R. Schroeder et al., J. Phys.:Condens.Matter 13, L313 (2001); R. Schroder et al., J. Chem. Phys. 116, 3449(2001)). Where TPA-dyes are doped in polymer matrices, again, the polymers are inactive compounds which do not take part in the up-conversion process.
Where the up-conversion is attributed to hot-band absorption, i.e. the excitation of molecules which populate high vibrational states, this has, in some cases been used for laser cooling of matter (J. L. Clark et al.Phys Rev. Lett 76, 2037 (1996)) and/or as a temperature probe of opto-electronic devices (J. M. Lupton, Appl. Phys. Lett 80, 186 (2002)).
Another area of research in the field of organic compounds is the field of “optical limiters”. An optical limiting material shows non-linear absorption characteristics. This is due to the phenomenon that the cross-section of the excited state is large than that of the ground state. The larger the input energy the more molecules become excited into the state having a larger cross-section, thus resulting in an increasing absorption coefficient. Optical limiters based upon this nonlinear absorption on the picosecond and nanosecond time-scales have been reported for a number of materials, including metallophthalocyanins, porphyrins and fullerenes but also inorganic nanoparticles (Qureshi F M et al. Chem. Phys. 231, 87 (1998) and other references cited therein; Sun et al. Int. Rev. Phys. Chem. 18(1) 43 (1999) and references cited therein; W. Su, T. M. Cooper; Chem. Mater. 10, 1212 (1998); J. W. Perry et al., Science 273, 1533 (1996); K. A. Nguen et al., J. Chem. Phys. 118, 5802 (2003.
None of these organic systems, however, have proved to be particularly versatile due to the inherent characteristics of the up-converting material present in the corresponding system. Furthermore in most cases up-conversion could only be induced under conditions of very low temperatures, −180° C. and below. Also, none of the optical limiting materials has been reported to show up-converting behaviour when this material is on its own.